const handleSubmit = async (form) => { if (isSubmitting) return; setIsSubmitting(true); const idempotencyKey = uuidv4(); try { const res = await axios.post('/records', form, { headers: { 'Idempotency-Key': idempotencyKey } }); showSuccess(res.data.id); } finally { setTimeout(() => setIsSubmitting(false), 500); } }; 7.2 Server: idempotency handling (pseudo-code)